What makes GM/Pennzoil Synchromesh (non-FM) work so well at stopping gear clashing is the magnesium-containing Friction Modifier that most people (quite understandably) don't realize that it has. What with it being referred to as "non-friction-modified", and all.
Have you ever seen so much confusion surrounding a gear lube? Uhhh... non-gear-lube? Unique gear lube? GLS gear lube but not for all GLS gear lube applications? Synthetic? Semi-synthetic? Paraffinic?
GM and Pennzoil Synchromesh are exactly the same. Except they are not. Pennzoil has the only license to make the stuff, and GM's is the same stuff in a different bottle. Except that the GM stuff is really made by Exxon-Mobil.
GM/Pennzoil Synchromesh shears severely, by 30+% over a 25k mile run. Except it thickens drastically, due to oxidation during use. Simultaneously, the 2000 ppm of P, 1200 ppm of Zn, and 3500 ppm of Mg all mysteriously disappear after the oil has thickened from 9.08 to 13.1 cSt, leaving exactly 0 ppm of each.
All these things I have heard from people claiming to be in the know. Isn't the Internet wonderful?